(1) Field of the Invention
This invention relates to a magnetic recording medium having a multi-layer film coating formed thereon and a process for making the same. More particularly, the present invention relates to a magnetic recording medium having a multi-layer structure capable of exhibiting an optimum recording density, and a process for making the magnetic recording medium.
(2) Description of the Related Art
Conventional magnetic recording disk mediums have a multi-layer structure composed of an undercoat, a magnetic layer, and a protective overcoat, which are in turn formed on a non-magnetic substrate by sputtering or other means. Further, a lubricating layer is formed as the outermost surface layer.
As the magnetic layer, cobalt alloys have been heretofore proposed which comprise cobalt as the primary ingredient and two or more metals selected from platinum, niobium, tantalum, chromium, nickel and other metals, and which are formed by sputtering using a target composed of the cobalt alloys (see, for example, Japanese Unexamined Patent Publication No. 61-292219, ibid. 62-239420 and ibid. 62-137720).
As the undercoat, chromium or chromium alloys such as a chromium nickel alloy are predominantly used (see, for example, Japanese Unexamined Patent Publication No. 62-257618, ibid. 63-106917 and ibid. 61-292219).
As the non-magnetic substrate, aluminum alloys such as aluminum-magnesium (AlMg) alloy, with a nickel-phosphorus NiP) surface coating, are predominantly used. In recent years, there is an increasing demand for a glass substrate and a carbon substrate instead of aluminum alloys because the surface of glass exhibits a good mar resistance and can be easily mirror-polished.
However, it is possible that part of the elements present on the surface of a glass substrate or a carbon substrate diffuses into an undercoat layer composed of chromium or a chromium alloy, and water vapor or hydrogen contained in micropores or grain boundaries within a glass substrate or carbon substrate is emitted, and further oxygen ions diffuse within a glass substrate. These phenomena tend to lead to recduction in the coercive force Hc and coercivity squareness ratio S* of the magnetic layer, and thus, the enhancement of magnetic recording density are hindered.
Therefore, a magnetic recording medium exhibiting enhanced coercive force Hc and coercivity squareness ratio S* in the coercive force-hysteresis curve is desired to enhance the magnetic recording density. This is especially true for a magnetic recording medium having a glass substrate or a carbon substrate.
It has been proposed in Japanese Unexamined Patent Publication No. 4-247323 that a titanium layer and a silicon layer are formed in this order on a non-magnetic substrate, and the thus-superposed layers are heated, followed by formation an undercoat, a magnetic layer, and a protective layer, whereby the magnetic characteristics are improved. However, it is possible that silicon tends to be present in excess on the surface portion of the superposed two layers, which reacts partially with chromium and exerts a harmful influence upon the stable orientation of chromium in the undercoat formed on the two layers.